22 Photos
Allophane
Chemical Formula	Al2O3·SiO21.3-2·H2O2.5-3
Species	Silicates
Mohs Scale	3
Color	White, pale blue to sky-blue, green, brown
Streak	White
Luster	Waxy
Diaphaneity	Transparent, Translucent
Cleavage	None ObservedNone
Fracture	Conchoidal
Crystal Habit：Rarely observed as ring-shaped particles, with diameters of 50 oA, which in three dimensions may represent sections through hollow spherules or polyhedra. As hyaline crusts and masses; stalactites and flowstones.
Geological Setting：A weathering product of volcanic ash. In hydrothermally altered igneous rocks, from the breakdown of feldspars, and in hydrothermal veins, typically related to copper deposits. In sedimentary rocks, including chalk and coal beds.

Allophane is a amorphous to poorly-crystalline hydrous aluminium silicate clay mineraloid. Its chemical formula is Al₂O₃·(SiO₂)_1.3-2·2.5-3(H₂O). Since it has short-range atomic order, it is a mineraloid, rather than a mineral, and can be identified by its distinctive infrared spectrum and its X-ray diffraction pattern. It was first described in 1816 in Gr?fenthal, Thuringia, Germany. Allophane is a weathering or hydrothermal alteration product of volcanic glass and feldspars and has a composition similar to kaolinite. It typically forms under mildly acidic to neutral pH (5-7). Its structure has been debated, but it is similar to clay minerals and is composed of curved alumina octahedral and silica tetrahedral layers. Transmission electron micrographs show that it is generally made up of aggregates of hollow spherules ~3-5 nm in diameter. Allophane can alter to form halloysite under resilicating aqueous conditions and can alter to form gibbsite under desilicating conditions. A copper containing variety cupro-allophane has been reported.

It forms waxy botryoidal to crusty masses with color varying from white through green, blue, yellow, to brown. It has a Mohs hardness of 3 and a specific gravity of 1.0.

It was named from the Greek allos - "other" and phanos - "to appear", as it gave a deceptive reaction in the blowpipe flame in old mineralogical testing.